A recent trend in aircraft design has been an increase in the amount of power conversion equipment connected to onboard generators. The increase in power conversion equipment is necessitated by an increase in utilities using the onboard generator as a power supply and has resulted in power conversion equipment using a higher percentage of the power output of the onboard generators.
Power conversion equipment typically has a low impedance value and can greatly increase the required output of the generator. A fault condition in the load can result in a large DC fault current being pulled from the generator. A fault current occurs when an electrical connection is created as a result of some breakage or an error. These electrical connections typically provide a direct connection to an electrically neutral component and result in a direct grounding path for the energy being pulled from the generator.
In conventional designs current transformers are used to measure the current produced by the generator. These measurements are then sent to circuit protection equipment such as a current limiter. The current limiter can be part of the generator control unit (GCU) or an independent component and typically contains protection functions for use in the case of a ground fault. A conventional multiphase generator controller detects the current being pulled from the generator and then adapts and controls the generator inputs accordingly.
One downside of the conventional design is that DC fault current will saturate the current transformers, effectively disabling the current limiter and associated protection. Different types of current sensors may be incorporated at additional cost and/or weight. Non-fault conditions can also produce DC currents. For example, AC motor start current contains some DC current for a short period. Such DC current should not be considered a fault condition. A failure to account for non-fault conditions that could appear to be fault conditions when examining a single phase could lead to an improper triggering of the circuit protection equipment. This improper triggering could result in an unnecessary shutdown of the generator or an associated controller.